Hall of Gemstone 
World Enchantment
$14.38 #13988 1.20 29.9 years
At the beginning of each player's upkeep, that player chooses a color. Until end of turn, lands tapped for mana produce mana of the chosen color instead of any other color.
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Cards similar to Hall of Gemstone expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Pulse of Llanowar73%
$0.28↓#26367↓0.37↓25.9 years↓
"If a basic land you control is tapped for mana, it produces mana of a color of your choice instead of any other type."
Orochi Leafcaller72%
$0.32↓#146100.33↓21.9 years↓
": Add one mana of any color."
: Add one mana of any color."
Signpost Scarecrow72%
$0.12↓#18588↓0.47↓6.9 years↓
"
: Add one mana of any color."
: Add one mana of any color."
Viridian Acolyte72%
$0.11↓#26078↓0.20↓22.6 years↓
", 
: Add one mana of any color."
, : Add one mana of any color."
Mana Cylix72%
$0.09↓#21797↓0.27↓25.6 years↓
", : Add one mana of any color."
, : Add one mana of any color."
Celestial Prism72%
$0.17↓#30168↓0.00↓33.1 years↑
", : Add one mana of any color."
, : Add one mana of any color."
Scarecrow Guide71%
$0.27↓#145600.00↓3.0 years↓
": Add one mana of any color. Activate only once each turn."
: Add one mana of any color. Activate only once each turn."
Salvaged Manaworker71%
$0.09↓#23985↓0.00↓4.0 years↓
": Add one mana of any color. Activate only once each turn."
: Add one mana of any color. Activate only once each turn."
Llanowar Envoy71%
$0.20↓#25907↓0.40↓8.4 years↓
": Add one mana of any color."
: Add one mana of any color."
Nomadic Elf71%
$0.14↓#27613↓0.29↓25.9 years↓
": Add one mana of any color."
: Add one mana of any color."
Helionaut70%
$0.15↓#31140↓0.00↓25.3 years↓
", : Add one mana of any color."
, : Add one mana of any color."
Harvest Mage70%
$0.09↓#29320↓0.00↓26.6 years↓
", , Discard a card: Until end of turn, if you tap a land for mana, it produces one mana of a color of your choice instead of any other type and amount."
, , Discard a card: Until end of turn, if you tap a land for mana, it produces one mana of a color of your choice instead of any other type and amount."